This happens when the JTAPI client is installed in the wrong directory. It should be installed in C:\Winnt\Java\Lib.
Solution:
1. Un-install the JTAPI Client from Add/Remove Programs (Control Panel)
2. Re-install the JTAPI Client from the Call Manager Plug-Ins.
3. Make sure you put in the C:\Winnt\Java\Lib directory when prompted for the destination of the Client Installation.